At present flow measurement cells are supplied virtually exclusively by mixing apparatus operating by the so-called "Stopped Flow" technique. A typical mixing apparatus of this kind is described for example in "Analytical Chemistry", vo. 48, No. 12, Oct. 1976, pp. 1686-1693. One disadvantage of such apparatus is that it comprises metering pumps, e.g. piston-type burettes, driven by a common motor for the liquids which are to be mixed together and consequently, unless the construction of the burettes is modified or the burettes are substituted, the mixing ratio cannot be altered once chosen.
A metering and diluting apparatus providing an adjusting mixing ratio has been proposed primarily for use in biomedical and clinical chemistry. With this apparatus mixtures (dilutions) can be produced having a dilution ratio which is variable within wide limits. This apparatus essentially comprises two piston burettes, controlled by stepping motors, which can be connected to supply vessels or to a mixing vessel by selecting the positions of two three-way valves. An electronic control means controls the burette motors and valves so that the volumes of the two liquids to be mixed which are to be fed into the mixing vessel are automatically taken up by the burettes. The volumes of liquid components are then expelled at a constant speed which can be adjusted manually.
This prior-proposed dilution apparatus is hardly used at all for directly supplying flow measurement cells. In fact, the use of the apparatus for this purpose would pre-suppose that the mixing ratio of the stream of liquid pumped through the measuring cell is constant, and that the volumes of liquid would have to be each expelled by the burettes at a constant speed and synchronously, i.e. at the same time. The burettes would therefore have to start at the same time and stop at the same time. Thus, different metering speeds would obviously be required for different mixing ratios.
These requirements are not satisfied, or can only be partly satisfied, by the prior dilution apparatus. This is particularly due to need for the metering speeds for each new mixing ratio having to be reset afresh by hand. Experience shows that this is very laborious and in any case only rarely proves successful. It is particularly difficult to adjust the speeds with the relatively high degree of precision often needed.
The same disadvantage is true of other known dilution apparatus such as described in U.S. Pat. Nos. 3,800,984 to Phelan and 3,982,667 to Chen, and in German (Federal Republic) Offenlegungschrift No. 19 50 770.
It is an object of the present invention to provide mixing apparatus for producing a stream of liquid comprising two or more components in which the stream contains components in a selected mixing ratio, the stream being homogeneous with respect to its components, and in which automatic control means is provided to control the mixing in accord with input data determining the required mixing ratio.
It is a further object of the invention to provide automatic control means for a mixing apparatus for liquid components that is operable to control the mixing ratio and to control valves and other devices for the take-up of the components for mixing in the required ratio.